Generally, transistors are formed from semiconductor materials such as silicon, germanium, or various compound semiconductor materials, and various transistors have been proposed so far.
FIGS. 1 through 12 show the examples of conventional transistors and diodes, wherein FIGS. 1 and 2 show the structure and the band structure of a typical bipolar transistor.
Referring to FIG. 1, the bipolar transistor includes an n-type emitter 11 surrounded by a p-type base 12, and a collector 13 surrounds the base 12 including the emitter 11. Under the collector 13, an n.sup.+ -type buried collector region 14 is formed, and the collector current obtained at the collector 13 is lead to the surface of the structure via the buried collector region 14 and further through an n.sup.+ -type contact region 15. Further, a substrate 16 of p-type silicon supports the foregoing vertical bipolar transistor structure. In correspondence to the emitter region 11, base region 12 and the collector contact region 15, an emitter electrode 17, a base electrode 18 and a collector electrode 19 are formed. As is commonly practiced, such bipolar transistor is formed by combining p-type and n-type silicon layers.
FIG. 2 shows the band structure of the bipolar transistor of FIG. 1. As can be seen therein, the n-type emitter region 11 has electrons, shown by solid circles in the drawing, in the conduction band as the carriers. On the other hand, the p-type base region 12 has holes, shown by open circles in the drawing, in the valence band thereof as the carriers. Further, the n-type collector region 13 has the electrons, also shown by the solid circles in the drawing, in the conduction band thereof as the carriers.
In operation, the base voltage at the base region 11 is controlled by the voltage that is applied to the base electrode 18. When a positive base voltage is applied, for example, the energy level of the base region 12 shifts in the downward direction. In response to such a downward shift of the base energy level, the potential barrier that has been formed by the conduction band of the base region 12 between the emitter region 11 and the collector region 13 substantially disappears. When this occurs, the electrons are injected from the emitter region 11 to the base region 12 and are transported through the base region 12 by the diffusion of the minority carriers. Thereby, the injected carriers reach the collector region 13 and a collector current flows through the transistor.
In such a bipolar transistor, the operational speed is mainly limited by the transit time of the carriers passing through the base region 12. Thus, in order to increase the operational speed of bipolar transistors, it is desired to form the base region by a semiconductor material having a high electron mobility. Alternately, the thickness of the base region 12 may be reduced for reducing the transit time of electrons through the base region. When the latter approach is employed, however, there arises a problem of increased lateral resistance of the base region. When this occurs, the voltage level within the base region is not controlled effectively and hence, the switching operation of the transistor. When the thickness of the base region 12 is reduced further, the depletion region accompanying the collector region is substantially overlapped with the depletion region that accompanies the emitter region. Such an overlapping of the depletion regions causes the so-called punch-through, wherein the flow of current through the base cannot be controlled anymore by the base voltage.
On the other hand, when the former approach is adopted, there is a limitation in the selection of suitable material, as the semiconductor materials other than silicon and germanium that can be doped to the p-type or n-type with controlled carrier concentration level, are relatively limited. Particularly, such conventional bipolar transistors cannot use the excellent electric properties of recently found high temperature superconductors.
FIGS. 3 and 4 show the structure of a conventional hot electron transistor and the band structure thereof.
Referring to FIG. 3, the device has an emitter region 21, a base region 23 and a collector region 25 each formed from a conductive material such as metal, and an emitter barrier region 22 and a collector barrier region 24 both of a wide gap semiconductor material are interposed between the emitter region 21 and the base region 23, and between the base region 23 and the collector region 25. The layered structure of the layers 21 through 24 is formed on a substrate 26, and the emitter region 21, the base region 23 and the collector region 25 have respective electrodes 17, 18 and 19.
In the hot electron transistor of such a construction, metals or doped semiconductors can be used for the emitter region, base region and the collector region, while insulating materials or semi-insulating materials can be used for the emitter barrier layer and the collector barrier layer. Further, such a structure does not require the exact control of the conductivity type for the emitter region, base region and the collector region.
As shown in the band diagram of FIG. 4, the emitter barrier region 22 and the collector barrier region 24 form the energy barrier against the passage of the carriers from the emitter region 21 to the collector region 25. With the increasing forward biasing across the base region 23 and emitter region 21, the barrier height of the emitter barrier 22 is decreased and the carriers in the emitter region start to cause tunneling through the region 22 to the base region 23. When there is a sufficiently large biasing across the collector region 25 and the emitter region 21, those electrons injected into the base region 23 and still maintaining the original energy pass through the base region 23 and reach the collector region 25. Thereby, a collector current flows in the transistor from the emitter region 21 to the collector region 25. The collector barrier 24 merely forms a potential barrier that separates the base region 23 from the collector region 25 electrically.
In such a hot electron transistor, however, there exists a problem in that, when the carriers injected to the base region 23 lose their energy by the scattering and the like, the carriers are confined between the barrier region 22 and the barrier region 24. Thus, the carriers thus trapped in the base region form a base current. In other words, the hot electron transistors tend to show an insufficient current gain.
FIG. 5 shows the band structure of a bipolar resonant tunneling transistor. The transistor basically has a structure of bipolar transistor and comprises an n-type GaAs emitter region 31, a p-type GaAs base region 32 and an n-type GaAs collector region 33. Between the emitter region 31 and the base region 32, there are formed a pair of resonant barrier region 34 of AlGaAs such that there is formed a two-dimensional quantum well between the two barrier regions 34. In operation, the barrier level of the base region 32 is changed in response to the base voltage, and the current flows from the emitter to the base upon the lowering of the barrier level of the base region 32.
In such a two-dimensional quantum well, a number of quantized states are formed with respective energy levels, and the incident carries that causes the resonance with the quantum level in the resonant barrier region 34 are selectively passed. Thus, the resonant tunneling transistor shows a strong non-linear characteristic that is not provided by other types of transistors.
In the band structure of FIG. 5, because of the lack of the collector barrier layer in contrast to the structure of FIG. 4, the problem of unsatisfactory common emitter current gain is eliminated. However, this structure still has the problem of transit time of carriers through the base similar to the transistor of FIG. 1 and the operational speed is limited. Further, the transistor requires a precise conductivity control for the collector, base and the emitter regions.
FIG. 6 shows the band structure of a resonant hot electron transistor. In contrast to the hot electron transistor of FIG. 3, the transistor of FIG. 6 has a resonant barrier region 36 for the base barrier region. Similar to the structure of FIG. 5, the resonant barrier region 36 comprises a pair of potential barriers that confine a two-dimensional quantum well therebetween. The resonant tunneling of the carriers occurs through the region 36 as a result of energetical resonance of the carriers with the quantum levels formed in the resonant barrier region 36. This transistor can use various metals and insulating materials and thus, the restriction about the materials that can be used for the transistor is relaxed. On the other hand, the transistor has the problem in that the common emitter current gain may be insufficient because of the trapping of the carriers in the base region 23.
FIG. 7 is a band diagram showing a tunnel transistor that has a structure of bipolar transistor. Referring to FIG. 7, the transistor comprises a p-type emitter region 41 and a p-type collector region 42 of GaAsSb wit an intervening base region 43 of n-type GaInAs. The thickness of the base region 43 is set small enough such that the carriers cause the tunneling from the emitter region 41 to the collector region 42 in response to a bias voltage applied to the base region 43. Typically, the thickness d of the base region 43 is set to about 50 .ANG..
GaAsSb forming the emitter region 41 and the collector region 42 has a composition of GaAs.sub.0.5 Sb.sub.0.5 with the carrier concentration level of 10.sup.16 cm.sup.-3 or less. On the other hand, the base region 43 has a high impurity concentration level and thus a high conductivity. Thereby, the potential barrier formed in correspondence to the base region 43 can be controlled in response to the base voltage applied to the base electrode. As the thickness of the base region 43 is thin, the reduced thickness of the barrier height causes immediately the tunneling of the holes from the emitter region 41 to the collector region 42, passing through the base region 43. The transit of the holes through the base region 43 by the tunneling is controlled by the base voltage and thus, the current flowing between the emitter and collector of the transistor is controlled by the base voltage. The time necessary for the holes to transit through the base region 43 is much smaller than the life time of the holes in the base region because of the reduced thickness of the base. Thus, the common base current gain of approximately 100% can be achieved by this conventional transistor.
This transistor, however, has a problem, associated with the extreme thickness of the base region 43, in that the base region 43 has an appreciable lateral resistance. When the base resistance is large, the time needed for controlling the potential barrier of the base region is increased because of the increased time constant formed as a result of the resistance and parasitic capacitance of the base region. Thus, the transistor generally fails to provide the high speed characteristics that is expected. The doping to reduce the lateral resistance of the base 43 is not successful, as such a doping causes an increased scattering of the holes in the base region and thus the reduced current gain. Further, this transistor also has the problem of limitation in the material that can be used for forming the active part thereof. For example, the high temperature superconductors that has been developed in the past several years cannot be employed.
Further, there is proposed a low temperature tunneling transistor as disclosed in the Japanese Laid-open Patent Application 60-142580, wherein the carrier transit time across a pair of adjacent electrodes is minimized by flowing the carriers in the form of tunneling current.
FIG. 9 shows the principle of this prior art transistor, wherein a GaAs channel layer 52 is provided on an insulating substrate 51, and source, gate and drain electrodes 53, 55 and 54 are provided on the channel layer 52. The channel layer 52 is formed to act as an insulator having the energy barrier of 2-3 meV in the low temperature region and this barrier is controlled in response to the gate voltage applied to the gate 55. FIG. 10 shows an improvement of the device of FIG. 9 wherein an insulating film 57 is interposed between the channel 52 and the gate electrode 55 for securing the channel-gate isolation.
This transistor of FIG. 9 and FIG. 10 has a serious problem, arising from the basic principle of the transistor, in that it has to satisfy two, mutually inconsistent requirements as follows. In the first requirement, to avoid the diversion of the tunneling current to the gate electrode and to maximize the current gain, it is necessary to separate the gate electrode 55 from the channel layer 52 as much as possible. In the second requirement, the gate electrode 55 has to be provided as close as possible to the channel layer 52 in order to achieve an efficient control of the barrier height in the channel layer 52 by the control voltage applied to the gate electrode 55. The separation between the gate electrode 55 and the channel layer 52 has to be set such that a tunnel current flows between the source 55 and the drain 54, and associated therewith, the gate 55 has to be provided closer to the channel layer 52 than the distance between the source electrode 53 and the drain electrode 54. However, such an arrangement inevitably causes the tunneling current flowing also through the gate electrode 55. The insulator film 57 of FIG. 10 is provided to avoid this problem. However, such an insulator film 57 decreases the potential coupling between the barrier in the channel layer 52 and the gate voltage. This problem may seem to be solved by setting a large distance between the source and drain electrodes and reducing the barrier height between the source and drain electrodes. However, the barrier height between the source and drain electrodes is determined by the contact potential between the source electrode 53 and the channel layer 52, and the contact potential between the channel layer 52 and the drain electrode 54. Thus it is generally not possible to realize the barrier height of 2-3 meV as is needed for this purpose.
FIG. 11 shows a typical conventional diode. Referring to FIG. 11, the diode comprises a cathode region 61 of n-type single crystal silicon and an anode region 62 of a p-type single crystal silicon contacted with each other to form a p-n junction. FIG. 12 shows the band structure of the diode of FIG. 11, wherein E.sub.F represents the Fermi level in the thermal equilibrium state, E.sub.C represents the bottom of the conduction band in the thermal equilibrium state, and E.sub.V represents the top of the valence band in the thermal equilibrium state. .PHI. represents the barrier height formed in the conduction band in the thermal equilibrium state.
In operation, a forward bias voltage is applied across the anode region 62 and the cathode region 61. In response to this, the Fermi level changes from E.sub.F to E.sub.F ', the conduction band from E.sub.C to E.sub.C ', and the valence band from E.sub.V to E.sub.V '.
In response to the application of the forward bias voltage, electrons are injected from the cathode region 61 to the anode region 62 as the minority carriers while holes are injected from the anode region 62 to the cathode region 61 as the minority carriers. Thereby a current flows across the diode. On the other hand, when the diode is reversely biased, the depletion region is formed at the p-n junction interface and the carriers do not flow across the p-n junction Thus, the diode shows the rectification operation.
The foregoing rectification is obtained by the existence of the barrier of which height .PHI. is determined by the band gap of the semiconductor material used for the cathode and anode. It should be noted that the barrier height .PHI. is decreased, when a forward biasing is applied, such that the current density in the diode is represented as EQU J=AT.sup.2 .multidot.exp(-.PHI..sub.o /kT)(exp(eV/kT)-1),
where A stands for the Richardson's constant T represents the absolute temperature, .PHI..sub.o represents the barrier height at the no bias voltage, k stands for the Boltzmann's constant, and V represents the forward bias voltage.
From this equation, it can be seen that the term eV has to have a value close to .PHI..sub.o in order that a sufficient forward current is obtained. In other words, there exists a forward threshold voltage in such a diode for causing to flow currents under the forward biasing state. This threshold voltage changes depending on the material and takes a value of about 0.6 volts in silicon and 0.2 volts in germanium. Thus, the conventional diode has required to have the input signal amplitude of at least 0.6 volts in the commonly used silicon diode. However, such a limitation excludes the possibility of rectifying signals such as the electromagnetic signals traveled over a long distance path or feeble radar echo signals from a distant target. It should be noted that such signals may have the signal amplitude of only 20 mV-30 mV in the maximum. In the case of the Josephson integrated circuits, a similar problem occurs. As the output logic amplitude of the Josephson processors is typically in the order Of 3 mV, the conventional diodes cannot process the output signal of the Josephson processors. As a matter of fact, there is no known diode that can rectify the output logic signal of Josephson processors directly.